1. Field of the Invention
The present invention concerns an x-ray radiator with an evacuated housing supported for rotation around a rotation axle, in which housing are arranged a cathode and an anode, whereby the cathode having a surface that emits electrons upon laser irradiation, and having a drive arrangement to rotate the housing.
2. Description or the Prior Art
High-power x-ray radiators typically have an anode that is supported such that it can rotate in order to ensure a high thermal resilience of the anode even during generation of x-rays with high radiated power.
DE 87 13 042 U1 describes an x-ray tube with an evacuated housing supported such that it can rotate around a rotation axis, in which are arranged a cathode and an anode. The cathode and the anode are permanently connected with the housing. The x-ray tube has a drive arrangement to rotate the housing around the rotation axis. A deflection system that is stationary with regard to the housing, deflects an electron beam emanating from the cathode to the anode such that it strikes an annular impact surface on the anode, whereby the axis of the annular impact surface corresponds to the rotation aids that proceeds through the cathode. Since the anode is connected in a heat-conducting manner with the wall of the housing, a high heat dissipation from the anode to the outer surface of the housing is ensured. Effective cooling is possible with a coolant that is admitted to the housing.
In this known arrangement, a relatively long electron flight path exists due to the axis-proximal position of the cathode and the axle-remote position of the impact surface of the anode. This causes problems in the focusing of the electron beam. Among other things, this problem occurs in the generation of soft x-ray radiation for which a relatively low voltage is applied between the cathode and the anode. Due to the lower kinetic energy of the electrons, a higher defocusing of the electron beam occurs, dependent on the space charge limitation. The usage of such an x-ray tube therefore is possible only in a limited manner in specific applications such as, for example, in mammography.
U.S. Pat. No. 4,821,305 describes an x-ray tube in which both the anode and the cathode are arranged axially symmetrically in a vacuum housing that can be rotated as a whole around an axis. The cathode is rotatably supported and has an axially-symmetric surface made of a material that emits photoelectrons during the incidence of light. The electron emission is initiated by a stationary light beam that is focused from outside the vacuum housing through a transparent window onto the cathode.
The ability to convert this concept to a practical device appears questionable, however, due to the quantum efficiency of contemporary photo-cathodes and the luminous power required. With the use of high luminous power, the cooling of the photo-cathode requires a considerable effort due to its rather low heat resilience. The surface of the photo-cathode is additionally subject to oxidation processes in the vacuum conditions realized in x-ray tubes, which limits the durability of such an x-ray tube.
U.S. Pat. No. 5,768,337 discloses an x-ray tube wherein, in a vacuum housing in which the photo-cathode and the anode are arranged, a photomultiplier is interposed between the photo-cathode and the anode. A lower optical power is thereby necessary for generation of x-ray radiation. The longer electron flight path with multiple deflections of the electron beam between the dynodes, requires a high expenditure for focusing the beam.
An x-ray scanner, in particular a computed tomography apparatus, is known from EP 0 147 009 B1. X-rays are generated by an electron beam striking an anode. Among other things, the possibility is mentioned to generate the electron beam by thermionically-emitted electrons by a cathode surface being heated by a light ray. The surface of the cathode should be capable of being rapidly heated and cooled by the disclosed embodiment of the cathode with a support layer made of a material with high heat conductivity. This appears to be problematic, however, with regard to the luminous power then required.
U.S. Pat. No. 6,557,851 describes a system for generation of therapeutic x-rays. Among other things, the possibility is generally mentioned that the electron beam necessary for the generation of x-ray radiation is emitted from a thermionic cathode heated by a laser.